Cat peeing inside her own house

Hy there, 
I have two cats, one male and a female, both living in an apartment, they are both castrated, with a difference of 8 months. She is the youngest and is 10 months old.
From two days ago, she started peeing inside her own house, I mean, first, she peed inside his house, just got inside and 5 min after, peed inside. And today, got inside own house and did it again.

The first peed made the male cat to urinate in his house some time after, the house had been washed and cleaned from the smell, now he is sleeping normally on it. The second one I had just noticed after smelling it, but being it humid, it had happened early the same day.

Apartment size, pretty big, 2 dorms, 1 bth, living room, kitchen, laundry room and a balcony. 

Stress? The two cats were raised together and never had any problems of this sort. They both have each its own litter box, actually one of those closed ones and use it with no problem at all. We clean it every day for stool and every month ( we use silica) all the contents are changed according to the product recomendation, or even before, depending on the usage.

My wife and I follow pretty much the same routine every day. Go to work in the morning, return in the evening, go to the gym, dinner and bed after some tv time with some time playing with the cats.

We spent three days away, in which they behaved nicely and were fed with no problem. It all happened two days after we arrived.
Is there something that can be going on? Nothing different on the neighborhood as a construction, or even new people on the building is disturbing from outside. 

What do you mean by in her own house? Do you mean she has a crate?

If she is about 10 months old, this is the age where she is growing up, so it would not surprise me to see her start to be more assertive. This is probably a dominance thing.

Yes, her own bed. She did it inside it. 

The point about dominance that I don't get is about her being castrated.
Still, it can be so. Just think it is , since the older cat, a male, has never done anything like it and they both go along pretty well.

Getting a cat spayed/neutered can cut down their aggression, but it doesn't mean they will never have dominance problems. Are their beds close together? Is one cat more or less affectionate than the other?

Both beds are in the living room, fairly close to each other, they even sleep together in cold days on the couch or in one of the beds. The male is more distant and independent, but still very affectionate and is always in the same room as we are. She is really active and engages in play sessions every day at night, specially catching a paper ball and returning it as a dog
They enjoy playing some "fight" with each other but never very aggressive, more like some chasing and some rolling on the floor. With some biting, never too hard.